Air poweres or electric a grinder/die grinder will be the best tool for cutting out rotten metal. Once you have used it for a while you may find that the guard will be in the way, just make sure you have your safety glasses and leather gloves on. The elctric grinders woork great and you don't have to buy a huge compressor to run it.
On another note the tool is only as good as the cut-off wheel, the wider/thicker the wheel the more material you'll be cutting and the slower the cut time. Wholesale Tool sells some great wheels - we us a 6"x.040" and a 4.5"x.035"( http://www.wholesaletoolcompany.com/?gclid=COORuKaT260CFYvRKgodkC3xmA ) on a 4.5" grinder and it works perfect. And this one in a die grinder http://www.wholesaletoolcompany.com/?gclid=COORuKaT260CFYvRKgodkC3xmA

....David